In the commercial nuclear power industry, there are primarily two types of reactor systems, namely boiling water reactors (BWR) and pressurized water reactors (PWRs). Both use water to moderate the speed of neutrons released by the fissioning of fissionable nuclei, and to carry away heat generated by the fissioning process. Water flows through the reactor core, is recycled, and inevitably becomes contaminated with iron, Fe-55, colloidal and soluble cobalt, Co-58, and Co-60, and other radionuclides. The water further becomes contaminated with organics (e.g., oils and greases), biologicals and non-radioactive colloids (e.g., iron rust).
In a boiling water reactor (BWR), the water passing through the core will be used directly as steam in driving turbine-generators for the production of electricity. In a pressurized water reactor (PWR), the primary water that flows through the reactor is isolated from the secondary water that flows through the turbine generators by steam generators. In both cases, while the chemical constituents of the waste water will be different, these reactor systems will produce colloidal, suspended and dissolved solids that must be removed before the waste water may be reused or released to the environment.
The presence of iron (as iron oxide from carbon steel piping) in Boiling Water Reactor (BWR) circuits and waste waters is a decades old problem. The presence and buildup of this iron in condensate phase separators (CPS) further confounds the problem when the CPS tank is decanted back to the plant. Iron carryover here is unavoidable without further treatment steps. The form of iron in these tanks, which partially settles and may be pumped to a de-waterable high integrity container (HIC), is particularly difficult and time consuming to dewater. The addition of chemicals upstream from the CPS, such as flocculation polymers, to precipitate out the iron only produces an iron form even more difficult to filter and dewater. Such chemically pretreated material contains both sub-micron particles and floc particles of sizes up to 100 microns. It is believed that the sub-micron particles penetrate into filter media, thus plugging the pores and preventing successful filtration of the larger micron particles.
Like BWR iron waste waters, fuel pools, or basins, (especially during the decontamination phase) often contain colloids which make clarity and good visibility nearly impossible. Likewise, miscellaneous, often high conductivity, waste steams at various plants contain such colloids as iron, salts (sometimes via seawater intrusion), dirt/clay, surfactants, waxes, chelants, biologicals, oils and the like. Such waste streams are not ideally suited for standard dead-end cartridge filtration or cross-flow filtration via ultrafiltration media (UF) and/or reverse osmosis (RO), even if followed by demineralizers. Filter and bed plugging are almost assured.
There are a number of prior art techniques used for removal of colloidal, suspended and dissolved solids, and the requirement to remove such materials from waste waters is not unique to nuclear reactors. However, the nature of nuclear reactors raises special concerns about the use of additives for chemical treatments because of the desire to avoid making radioactive wastes also chemical wastes.
There are other concerns as well. The processed waste water must be quite free of radioactive contaminants if it is to be released to the environment. The radioactive material extracted from the waste water during processing must be stable or in a form that can be stabilized for disposal in a way that meets disposal site requirements, particularly with respect to preventing the leaching out of radioactive contaminates by liquid water. Finally, the volume of the waste must be minimized because of both the limited space available for disposal of radioactive waste and the high cost of its disposal.
Accordingly there is a need for better ways of processing radioactive waste water containing suspended solids and dissolved ions from nuclear power reactors and other sources.